October 12, 2022
Those ancient planetary drives I first got involved with in 1979 (see previous 7/3 posting) were a collection of very simple to make parts. The double-helical designs of thirty years later were conceptually the same but dauntingly complex to manufacture — sort of like parking a Model T Ford next to a modern hybrid super car. Both have the same objective. The “primitive” drives used simple through hardened spur gears. The planets — usually three, but occasionally four to comply with the “rules” for assembly—rotated on hardened pins via bronze bushings. Those pins were press-fitted into a thick steel plate to obtain a cantilever mounting. Nothing had to be aligned or welded. The “advanced” design required ground tooth helicals to be electron beam-welded in special fixtures. Those sub-assemblies had to be finish machined before anti-friction bearings were installed. Highly engineered “straddle mount” planet carriers were needed, along with careful installation of the heated planets around the “captured” sun pinion, using frozen planet pins to obtain a tight fit on the pins. On the old spur units, the internal gears were keyed to the cast ring of the housing and held in place axially with set screws. The double-helical internals required multiple dowel pins for alignment and had to be assembled over the by now very heavy carrier assembly. Unless you took great care, it was easy to get the teeth misaligned, causing rework and delay. So why did we do this to ourselves? Think back to the relative power density of that Model T Ford compared to a modern direct-injected turbo charged four-cylinder. The T made 25 horsepower from over 3 liters of displacement. The modern motor would push out 10x as much; current epicyclic gearboxes provide the highest torque capacity per-unit-volume, and people are willing to put up with the manufacturing and assembly complexities to save space and weight.